1 ========================
2 Segmented Stacks in LLVM
3 ========================
11 Segmented stack allows stack space to be allocated incrementally than as a
12 monolithic chunk (of some worst case size) at thread initialization. This is
13 done by allocating stack blocks (henceforth called *stacklets*) and linking them
14 into a doubly linked list. The function prologue is responsible for checking if
15 the current stacklet has enough space for the function to execute; and if not,
16 call into the libgcc runtime to allocate more stack space. When using ``llc``,
17 segmented stacks can be enabled by adding ``-segmented-stacks`` to the command
20 The runtime functionality is `already there in libgcc
21 <http://gcc.gnu.org/wiki/SplitStacks>`_.
23 Implementation Details
24 ======================
26 .. _allocating stacklets:
31 As mentioned above, the function prologue checks if the current stacklet has
32 enough space. The current approach is to use a slot in the TCB to store the
33 current stack limit (minus the amount of space needed to allocate a new block) -
34 this slot's offset is again dictated by ``libgcc``. The generated
35 assembly looks like this on x86-64:
43 # More stack space needs to be allocated
44 movabsq $8, %r10 # The amount of space needed
45 movabsq $0, %r11 # The total size of arguments passed on stack
47 ret # The reason for this extra return is explained below
49 # Usual prologue continues here
51 The size of function arguments on the stack needs to be passed to
52 ``__morestack`` (this function is implemented in ``libgcc``) since that number
53 of bytes has to be copied from the previous stacklet to the current one. This is
54 so that SP (and FP) relative addressing of function arguments work as expected.
56 The unusual ``ret`` is needed to have the function which made a call to
57 ``__morestack`` return correctly. ``__morestack``, instead of returning, calls
58 into ``.LBB0_2``. This is possible since both, the size of the ``ret``
59 instruction and the PC of call to ``__morestack`` are known. When the function
60 body returns, control is transferred back to ``__morestack``. ``__morestack``
61 then de-allocates the new stacklet, restores the correct SP value, and does a
62 second return, which returns control to the correct caller.
64 Variable Sized Allocas
65 ----------------------
67 The section on `allocating stacklets`_ automatically assumes that every stack
68 frame will be of fixed size. However, LLVM allows the use of the ``llvm.alloca``
69 intrinsic to allocate dynamically sized blocks of memory on the stack. When
70 faced with such a variable-sized alloca, code is generated to:
72 * Check if the current stacklet has enough space. If yes, just bump the SP, like
74 * If not, generate a call to ``libgcc``, which allocates the memory from the
77 The memory allocated from the heap is linked into a list in the current
78 stacklet, and freed along with the same. This prevents a memory leak.